langgraph: fix add_node input schema error

[gbaian10]

fix: #1331

I used inspect.signature to read the name of the first parameters and determine whether this variable has a type_hint.

And used StopIteration to prevent situations where there are no parameters.
Although in this situation, executing invoke will still result in an error.

If want to catch errors before running the graph, maybe you can check that the number of parameters is not zero?

[gbaian10]

Regarding first_parameter_name = next(iter(inspect.signature(action).parameters.keys()))

Also can use list() and then check the length before accessing [0].

I'm not sure which one is better.

[nfcampos]

Thanks, do you want to add a test case?

[gbaian10]

Thanks, do you want to add a test case?

I plan to add tests using this logic. Do you have any better suggestions?

from typing import TypedDict

from langchain_core.runnables import RunnableConfig
from langgraph.graph import StateGraph

# from libs.langgraph.langgraph.graph.state import StateGraph


class InputState(TypedDict):
    question: str


class OutputState(TypedDict):
    input_state: InputState


def foo(state: InputState) -> OutputState:
    return {"input_state": state}


def bar(state, config: RunnableConfig) -> OutputState:
    # The expected `state` result is `InputState`, but received `RunnableConfig`.
    return {"input_state": state}


def baz(state, config) -> OutputState:
    # The expected `state` result is `InputState`, but received `OutputState`.
    return {"input_state": state}


graph = StateGraph(input=InputState, output=OutputState)
graph.add_node(foo)
graph.add_node(bar)
graph.add_node(baz)

graph.set_entry_point(foo.__name__)
graph.add_edge(foo.__name__, bar.__name__)
graph.add_edge(bar.__name__, baz.__name__)
graph.set_finish_point(baz.__name__)

graph = graph.compile()

for v in graph.stream({"question": "Hello"}, stream_mode="values"):
    print(v)
    if "input_state" in v:
        assert v["input_state"] == {"question": "Hello"}

[vbarda]

Thank you @gbaian10 -- going to merge!